Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.
Meetup - Helps groups of people with shared interests plan events and facilitates off line group meetings in various localities around the world.
wikiHow - wikiHow is a wiki based collaboration to build the world’s largest, highest quality how to manual.
Eventbrite - Discover Great Events or Create Your Own & Sell Tickets
AllAnswered - Your Team's Collective Brain All-In-One Knowledge Management System Wiki + Q&A + Workflow + AI
Bigtent - BigTent is a web-based organization and communication portal for groups.